Optical defect inspection systems have been used to identify defects in articles such as fruits and vegetables, processed meats, baked goods, and nonfood items. These inspection systems typically employ laser-based scanning or charge-coupled device-based video cameras to detect undesirable variations in the shade of the article passing through a scanning area. In some inspection systems, inspection equipment create shadows which may be mistaken for defects or otherwise make identification of defects unreliable.
Shadowless performance has been obtained by using many, for example, twenty-six, incandescent lamps in a generally spherical arrangement around the articles being inspected. The lamps are used in connection with reflectors, iris apertures, and projection lenses. Shadowless illumination of the articles positioned at an image plane requires adjustment and alignment of the lamps with the apertures until cameras view even illumination from all sides of a white test target. In addition to the twenty-six incandescent lamps, the system uses two fluorescent lamps that provided a saturated background having a brightness over 10% above the brightness of a white test object in the image plane.
The incandescent lamps of the prior system are operated at a regulated voltage that is much lower than their normal operating voltage in order to maintain a relatively stable light output and stable spectral distribution over a reasonable period of time. Despite the low operating voltage, the twenty-six lamp system requires 540 watts to produce a light level of 500 footlamberts over a seven inch (178 millimeter) wide image plane. However, the lamps become uncalibrated with time because of lamp aging. There has been no way to keep the light output on the article calibrated other than the regulation of the DC power supply that powers the lamps. Because of the above-mentioned characteristics, these prior systems have been expensive to build and operate and difficult to maintain.